Loss of endothelial function is a critical factor in the development of coronary artery disease (CAD). Still, endothelium dependent signaling and particularly the underlying physiological mechanism responsible for endothelial control of coronary artery tone are poorly understood. We have recently demonstrated that small conductance Ca2+-activated K+ channels, SK3, are fundamental controllers of endothelial function in peripheral arteries, promoting sustained dilation proportional to their level of expression. Our preliminary data in coronary arteries indicate that under physiological pressure and flow, constitutive SK3 channel activity opposes steady-state tone. We have now identified repetitive spontaneous Ca2+ transients generated in coronary endothelial cells that may provide the fundamental stimulus for SK3 channel activation. In particular, the proximity of these Ca2+ signals to holes in the internal elastic lamina (IEL) as well as to SK3 channels and specific gap junction connexins expressed along the endothelial-vascular smooth muscle (VSM) interface, suggest the existence of a localized myoendothelial signaling complex. We hypothesize that under physiological conditions in coronary arteries, repeated Ca2+-dependent activation of membrane SK3 channels drives tonic endothelial hyperpolarization, which is rapidly communicated through IEL holes to adjacent smooth muscle through gap junctions. Moreover, we propose that estrogen-induced upregulation of endothelial SK3 channel expression amplifies this endothelium-derived hyperpolarization (EDH). To fully address this hypothesis, we have formulated two specific aims. Aim 1 will directly assess the functional coupling of spontaneous Ca2+ events to SK3-dependent membrane potential hyperpolarization in the endothelium of intact coronary arteries and whether this effect is altered by differential SK3 expression (i.e. via direct genetic manipulation or estrogen) and shear stress. Aim 2 will assess whether focal Ca2+-dependent SK3 activation at sites of IEL holes allows for direct hyperpolarization of subintimal VSM via gap junctions, thereby reducing VSM Ca2+ and promoting coronary artery dilation. In pursuit of these aims, we will apply state-of-the-art and innovative approaches including 1) simultaneous high-speed confocal imaging and intracellular electrophysiological measurements in intact coronary arteries, 2) a unique genetic mouse model (SK3T/T) in which SK3 expression can be experimentally controlled to unequivocally discern the specific impact of SK3 channels, and 3) novel peptide inhibitors to target specific gap junction connexins. This work will provide a paradigm of fundamental endothelial signaling and physiological vasoregulation in coronary arteries, and identify potential cellular targets for future therapies against CAD.